The accuracy of ac measuring instruments such as voltmeters or oscilloscopes is affected by the stability of the components used therein. For instruments whose accuracies are on the order of one tenth of one percent or better, the variable capacitors used to adjust the compensation of the input attenuator need to be extremely stable if the instrument is to stay calibrated for any appreciable length of time. Changes in temperature or humidity and aging are among those things that can alter the values of capacitors in an attenuator and cause changes in frequency response that can severely degrade accuracy.
In principle, an attenuator can be recompensated at any time. Sometimes this is actually practical, as in the familiar case of compensating the external attenuator probe of an oscilloscope. But there are at least two reasons for the ease with which that can be done. First, it doesn't need to be done with any great regularity. One would be fortunate to make a one percent measurement with an oscilloscope, let alone a one tenth of one percent measurement. So the stability of the trimmer and other capacitors involved seems high since they are adaquate for the measurement environment in which they are being used. Next, the adjustment itself is physically easy to perform since the trimmer to be varied is readily accessible on the probe itself. It is neither part of a main input attenuator that is located behind the front panel nor otherwise buried within the innards of the instrument.
In practice, however, it may not be practical to frequently recompensate an internal attenuator when the required accuracy is considerably greater and the physical placement of the trimmer cannot, for whatever reason, be upon the front panel. Consider the problem of compensating the internal attenuator of an ac voltmeter accurate to one tenth of one percent. First of all, the stability of the trimmer becomes a very significant concern. A change in capacitance of even one one hundredth of a picofarad can be intolerably large. Next, it may be difficult to avoid the introduction of unwanted reactance in the compensation network if it includes a component for front panel adjustment when the rest of the attenuator is some distance away. And finally, in high accuracy applications a real problem of user convenience begins to arise. If the instrument frequently needs to have the attenuator recompensated then that extra overhead can become a burden. It would be desirable if compensation could be done as frequently as needed without any inconvenience that might otherwise discourage necessary compensation. Such inconvenience could easily lead to erroneous measurements by an inattentive operator. Automatic and rapid compensation would be a desirable feature, especially if it could be accurately done at low cost. Since many measurement instruments now also incorporate microprocessors for other purposes anyway, it would also be desirable if such an automatic compensation scheme relied upon a microprocessor to the greatest extent possible to thus minimize the need for additional hardware mechanisms within the instrument.